EP0363943A2 - Method for forming halftone data - Google Patents
Method for forming halftone data Download PDFInfo
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- EP0363943A2 EP0363943A2 EP89118918A EP89118918A EP0363943A2 EP 0363943 A2 EP0363943 A2 EP 0363943A2 EP 89118918 A EP89118918 A EP 89118918A EP 89118918 A EP89118918 A EP 89118918A EP 0363943 A2 EP0363943 A2 EP 0363943A2
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- halftone
- data
- screen
- signals
- image signals
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
- H04N1/46—Colour picture communication systems
- H04N1/52—Circuits or arrangements for halftone screening
Definitions
- This invention relates to a method for forming halftone data for flat bed type color scanners or the like, and more particulary to a method for forming the halftone data which takes into consideration the difference in halftone screen percentage of respective dots in order to avoid a tone jump when image signals are obtained by scanning an original comprising color images in continuous tone, and are superposed with halftone screen signals which are electrically generated so as to form four-color separeted halftone gradation images in C (cyan), M (magenta), Y (yellow) and K (black).
- Such an image scanning/reading/recording system comprises basically an input section, a controller and an output section. More particularly, the system includes an input section where image signals are picked up by the illumination, color separation and photometric systems to convert image information photoelectrically, and the controller which processes the information in arithmetic operation such as gradation correction, color correction, contour emphasis, conversion from R (red), G (green), B (blue) ⁇ C,M,Y,K and the like depending on the plate-making conditions.
- the processed image information is converted into optical signals such as laser beams by the output section to be printed in the form of images on a recording medium comprising a photosensitive material.
- the medium is developed by a predetermined developing device and used for printing and so on as the original film plates.
- the original to be printed is a photograph or a painting of continuous tone images
- the original should be divided into dots to express the density of images.
- the continuous tone images are therefore transformed into halftone gradation images which are a group of dots in the sizes which are different depending on the density thereof.
- As a method for forming such dots there has been proposed a method which irradiates optical signals depending on the continuous tone images on a recording medium via a contact screen placed on a film.
- the contact screen comprises arrays of dots with blurred circumferences.
- the reference numeral 100 denotes a basic periodic section of the halftone screen which is electrically formed.
- the halftone screen formed by repetition of the same pattern, and the minimum unit thereof is the basic periodic section 100.
- the basic periodic section 100 comprises eight scanning lines S1 through S8 arranged in parallel to each other in the direction of Y axis.
- the scanning lines S1 through S8 form respectively parts of the basic periodic 100 with unique voltage signals which change along the recording direction X.
- the respective voltages of the scanning lines S1, S2, S4 and S5 are set at a high level when they are passing through a point A through a point D in a dot section 101 while the voltage of the scanning line S3 is set at a low level when it passes through the point E.
- the voltages of the respective scanning lines S1 through S5 are set to gradually decrease from the points A through D toward the point E.
- the voltage signals at the scanning lines S1 through S8 may be formed into the halftone screen signals by, for example, superposing plural alternating voltage signals of a triangle shape of different periods and gradually shifting their phases for each scanning line.
- the halftone screens are formed respectively in the form rotated from the recording direction X at a predetermined angle ⁇ in order to prevent generation of Moiré patterns at the time of superposing.
- a basic periodic section 100 which is formed as above is generated periodically at a frequency sufficient to cover the scanning area of the original to form the halftone screen.
- the halftone screen signals forming such a halftone screen are superposed with the image signals which are optically read from the original at the input sectin of the image scanning/reading/recording system in order to form the halftone gradation images on the original film plate.
- a halftone plate of an area modulation type for color printing is characterized by the number of screen lines (e.g. the number of lines/inch: LPI), the screen angle (m/n) and the dot patterns.
- the screen angle is a rational number defined by m/n of FIG.1, and is required for each of four colors of the four colors of C,M,Y and K.
- the screen angle and the dot patterns are formed with the method disclosed in Japanese Patent Publication No.49361/1977 mentioned above.
- An arbitrary number of the screen lines may easily be obtained in the drum type color scanners simply by changing the imaging magnification with optical zooming, but in the flat bed (plane) type color scanners, the optical zooming is quite difficult.
- This invention was contrived to eliminate such conventional problems and aims to provide a halftone data forming method for flat bed type color scanners which can change the number of screen lines arbitrarily without changing the scanning pitch or the size of the light spot and which can prevent generation of the tone jumps.
- a method for forming halftone data which is characterized in that when an original comprising color images of a continuous tone is scanned to obtain image signals, and the image signals are superposed with halftone screen signals which are electrically generated in order to form multicolor separated halftone gradation images which are reproducible by printing, the difference in dot percentage between a first contact and a second contact in square dots is about 2% or higher.
- a method for forming halftone data which is characterized in that when an original comprising color images of a continuous tone is scanned to obtain image signals, and the image signals are superposed with halftone screen signals which are electrically generated in order to form multicolor separated halftone gradation images which are reproducible by printing, the difference in dot percentage between one stage of dot where blackening starts and another stage or where whitening ends is about 2% or higher.
- this invention when separated halftone gradation images of C,M,Y and K are being formed, a light spot of predetermined size is exposed to light and scanned while the number of pitches is being restricted in order to obtain an arbitrary number of the screen lines. Since the number of the screen lines is defined with the number of pitches, this invention method may easily be applied to color scanners of the flat bed type with conventional screen angles and dot patterns.
- halftone screen signals or data of a dither matrix are re-arrnged in order to prevent tone jumps when the images are outputted.
- angles of the four plates of C,M,Y and K are selected to be 0° , 15° , 45° and 75° , and the Y-plate is allotted at the angle 0° , and the other three plates of C,M and K are allotted at the remaining three angles respectively. It is known emperically that the Moiré pattern by the Y-plate is not visually recognizable, and therefore the Moiré pattern caused by the remaining plates of C,M and K, should be dealt with.
- m , n and a are integers, and the numerical values are given in a manner to satisfy the following formula or the conditions to dissolve a single Moiré pattern disclosed in Japanese Patent Laid-open (kokai) No. 188564/1987 wherein ⁇ is an integer.
- the primary Moiré pattern produced by the two plates of 45° ⁇ (30° ⁇ ⁇ ) (provided, however, the angles 15° and 75° are nominal angles) takes place precisely at 45° .
- the sufficient condition to prevent occurrence of the secondary Moiré pattern is the condition to the period of the primary Moiré pattern agree with the dot interval of the third plate of 45° .
- the line interval d15 at 15° becomes as expressed in equation (12) below.
- the real angle ⁇ 15 for 15° is expressed as below.
- Those marked with asterisk "* " represents Moiré patterns with relatively longer periods while the rest represent the conditions where no Moiré pattern is formed. Even with the conditions marked with "* " , they would not present a significant problem in practice as their period is long.
- the number of screen lines of 150 and 120 lines can be obtained by using 12.5 ⁇ m as the scanning pitch or 10/9 times of 11.25 ⁇ m.
- the coarse screen line numbers 65 and 85 use the pitch of 11.25 ⁇ m which is twice as much as 11.25 ⁇ m. If the scanning pitch P is selected to be 2.25 ⁇ m, then arbitrary number of the screen lines may be selected with four colors without Moiré patterns, but the realization of the scanning pitch 2.25 ⁇ m has too problems.
- One problems lies in the difficulty to focus the light beam (light spot) at 2.25 ⁇ m and the other problem is that the scanning speed becomes too slow with the scanning pitch of 2.25 ⁇ m while the data capacity increases remarkably. Because of the above two reasons, selection of a scanning pitch as small as 2.25 ⁇ m is not suitable.
- FIG.5 shows the structure of a system which reads images with a color scanner or the like, processes them in halftone and records them in images. More particulaly, the image signals read by an image input section 100 are digitized by an A/D converter 101, processed for gradation conversion, contour emphasis and so on by a signal processing section 102 and inputted to a dot converting section 103. The image data which are converted into dots by the dot converting section 103 are inputted to a D/A converter 104, converted into analog values and recorded in images by an image output section 105.
- FIG.6 shows the light beam scanning apparatus 30 comprises a laser diode 32 which outputs laser beam L1 for synchronization under the control of LD driver 31, and a laser diode 34 which outputs laser beam L2 for recording under the control of an LD driver 33.
- the laser beam L1 for synchronization which is outputted from the laser diode 32 is directed to a galvanometer mirror 40 via a collimator 36 and a mirror 38.
- the laser beam L2 for recording which is outputted from the laser diode 34 is directed to the galvanometer mirror 40 at an angle ⁇ in respect of the laser beam L1 for synchronization via a collimator 42 and the mirror 38.
- the galvanometer mirror 40 deflects the laser beam L1 for synchronization and the laser beam L2 for recording through reflection when the mirror is vibrated at a high speed.
- the laser beams L1 and L2 which have been reflected and deflected by the galvanometer mirror 40 are directed respectively to a synchronization signal generator 50 and an image recording section 52 via a scanning lens 48 comprising an f ⁇ lens.
- the synchronizing laser beam L1 enters the scanning lens 48 at an incident angle of ⁇ in respect of the optical axis and directed to the synchronizing signal generator 50 via the peripheral side of the scanning lens 48.
- the recording laser beam L2 enters the scanning lens 48 within the plane including the optical axis and directed to the image recording section 52 via the center of the scanning lens 48.
- the synchronizing signal generator 50 includes grids 56 which are formed with a large number of slits 54 at uniform intervals in the scanning direction of the synchronizing laser beams L1, and the synchronizing laser beams L1 is directed into the grids 56 via a mirror 58.
- a light condensing rod 57 is provided on the back surface of the grids 56, and the synchronizing laser beam L1 is directed to photodetectors 60a and 60b on both sides of the rod via the light codensing rod 57 and converted into electric signals.
- the electric signals from the photodetectors 60a and 60b are multiplied by a PLL (Phase Locked Loop) multiplier 62 and supplied to an output controller 64 as a synchronizing signal.
- the output controller 64 controls the LD driver 33 based on the synchronizing signals and image signals. As shown in Table 2, once the number of screen lines and screen angles are determined, the step proceeds to designing a dot pattern.
- FIG.7 shows a dot pattern ( template or dither matrix ) of 0° with 175 line.
- dot patterns template data as shown in FIG.7 or in practice as shown in FIG.8 are designed and used for formation of the halftone screens.
- the template data are usually stored either in a floppy disc or an ROM of the image output section 105, and at the time point when the number of screen lines and the screen angle are selected, a scanning pitch p is selected from the table and a circuit is set to be controlled with the selected pitch p .
- the data of the template data is read out in an RAM region.
- halftone data 3 are formed.
- the template data 1 which are read out are compared to the image data 2, and when the image data 2 are larger, "1 " is outputted, and when they are smaller, "0 " is outputted.
- the halftone data 3 is obtained with which light is modulated to expose a photosensitive material to obtain a halftone plate.
- the method for forming a halftone screen by the dot converting section 103 is described below.
- FIG.10 shows an example of the halftone screen signals wherein a basic periodic section 10 of a halftone screen comprising 100-halftone data is shown.
- the basic periodic section 10 is defined with the points A through D to be a halftone section 12 at a screen angle ⁇ .
- n ⁇ L ⁇ ⁇ P (23) wherein n ⁇ L denotes a length either in the X direction or the Y direction of the basic periodic section 10 when a side of a square forming the halftone section 12 in either the X direction or the Y direction is denoted as L; P denotes a width of the halftone block formed with a halftone data, and ⁇ denotes the number of the halftone data forming the basic periodic section 10 in either the X direction or the Y direction .
- the basic periodic section 10 can be structured by repeating generating the halftone data between the points A and B in the direction Y.
- FIG.11 shows an embodiment of a circuit which is used to generate the halftone gradation image signals.
- the circuit comprises two counters 14 and 16, an address converter 18, a halftone data (dither matrix data) memory 20, a line memory 22 and a binary signal generator 24. Out of preset plural halftone original data 26 selected desirable ones depending on the level of the halftone resolution and the screen angle ⁇ to be transferred and stored in the halftone data memory 20.
- the counter 14 counts the clock signal ⁇ x in the main scanning direction of the continuous tone images and supplies them as an address signal X to the address converter 18.
- the counter 16 counts the clock signal ⁇ Y in the auxiliary scanning direction of the continuous tone images and supplies them to the address converter 18 as an address signal Y.
- the address converter 18 converts the address signals (X, Y) of the halftone data at the basic periodic section 10 into the address signals (x, y) of the halftone data a0 through a19 stored in the halftone data memory 20 and is structured as shown in FIG.12. More specifically, the address converter 18 includes a remainder calculating section 183 to which the address signal X is supplied via an adder 182 and a remainder calculating section 184 to which address signal Y is supplied.
- the adder 182 is supplied with an adding signal F(Y) selected by the address signal Y from an off-set table 181.
- the off-set table 181 comprises plural data which are determinable by combination of the screen angle ⁇ and the halftone resolution level similarly in the case of the halftone original data 26.
- the remainder calculating section 183 calculates the address signal x based on the equation below.
- x MOD( X + F(Y), N x ) (27) wherein N x denotes the number of the halftone data in the direction X
- the remainder calculating section 184 calculates the address signal y based on the fonmula below.
- y MOD(Y,N y ) (28) wherein N y denotes the number of the halftone data in the direction of Y.
- the line memory 22 is supplied with the continuous tone image signals and the clock signal ⁇ x in the main scanning direction, and the outputs from the line memory 22 and the halftone data memory 20 are supplied respectively to the binary signal generator 24.
- the binary signal generator 24 compares the halftone data with the image signals and outputs the results of comparison as the halftone gradation image signals.
- a method to convert the continuous tone image signals into the halftone gradation image signals by using the basic periodic section 10 shown in FIG.10 is described below.
- Halftone data of various levels of the screen angle ⁇ and the halftone resolution are stored in the halftone original data 26. Out of these data, the halftone data a0 through a19 are selected and loaded at the address (x, y) in the halftone data memory 20.
- the address (x, y) is set within the range (0, 0) through (9, 1). Therefore, when an address signal (X, Y) from the counters 14 and 16 based on the clock signals ⁇ x and ⁇ y are inputted to the addres converter 18, the address converter 18 converts the address signals (X, Y) into the address signals (x, y) of the halftone data a0 through a19.
- the address converter 18 selects the halftone data a4 out of the halftone data a0 through a19 stored in the halftone data memory 20, and supplies the data to the binary signal generator 24. Similarly, all of the halftone data a0 through a19 which form the basic periodic section 10 are sequentially supplied to the binary signal generator 24.
- the binary signal generator 24 is supplied with the continuous tone image signal based on the clock signal ⁇ x from the line memory 22 in addition to the halftone data a0 through a19.
- the binary signal generator 24 compares the halftone data a0 through a19 with the continuous tone image signals, and outputs the result of the comparison in the form of the halftone gradation image signals or ON/OFF signals.
- the halftone signals are converted into optical signals such as laser beams and irradiated on a film by a device shown in FIG.6 to become the halftone gradation images.
- a tone jump tends to occur usually at near 50% in a square dot. This is attributable to the fact that the scanning beam has an extensive width, and therefore the tone jump tends to occur at a level from which blackened portions start to be coupled between adjacent dots.
- the ratio of the blackened portion increases in the process FIGs.14A ⁇ 14B ⁇ 14C or FIGs.16A ⁇ 16B ⁇ 16C ⁇ 16D ⁇ 16E.
- the dither matrix is formed as shown in FIG.19 with difference in dot percentage between the first and the second contacts by first connecting one contact and then the other contact as shown in FIG.15B to form a square pattern as shown in FIG.15C.
- the required difference in dot percentage is about 2% or more.
- the dot shapes at various levels of the halftone data will become as shown in FIGs.17A through 17E when the dither matrix is used.
- the tone jump tends to occur at the level where the blackening starts in the main scanning direction. This is because due to the extensive width of the scanning beam, the dot area does not increase smoothly.
- the tone jump were likely to occur in the process of (A1) ⁇ (A2) or (A2) ⁇ (A3) in FIG.20.
- P1 through P5 in FIG.20 show respectively the minimum unit of dot structure.
- FIG.21 shows the dither matrix wherein the difference in dot ratio between the dots where the blackening starts in the main scanning direction or between dots where the whitening ends is 2% or higher to prevent the tone jumps.
- the tone jumps tended to occur at the transitions from (B1) ⁇ (B2) and (B3) ⁇ (B4) in FIG.20, but as the levels of (B1) ⁇ (B2) and (B3) ⁇ (B4) were separated to a certain extent, the number of the tone jumps was less than the dither matrix in FIG.8 as the whole halftone gradation. As to the whitened portion, the similar fact applid only if the relation of negative vs. positive is reversed.
- the halftone data assumes the forms as shown in FIG.18A through FIG.18E at various levels of the halftone data when the dither matrix of FIG.21 was used.
- the letter (A) represents the level where the blackened pixels exist in the number of 1 to 4, (B) in the number of 1 to 5, (C) in the number of 1 to 13, (D) in the number of 1 to 19, and (E) in the number of 1 to 25.
- this invention method for forming halftone data can provide a desirable number of screen lines by exposing and scanning a predetermined size of a light spot at a predetermined pitch and by controlling the number of pitches when a multi-colored separated halftone gradation images are formed by combining all of C,M,Y and K or any two or more colors thereof, and prevent occurrence of the tone jumps by re-arranging the dither matrix or screen signals there of.
Abstract
Description
- This invention relates to a method for forming halftone data for flat bed type color scanners or the like, and more particulary to a method for forming the halftone data which takes into consideration the difference in halftone screen percentage of respective dots in order to avoid a tone jump when image signals are obtained by scanning an original comprising color images in continuous tone, and are superposed with halftone screen signals which are electrically generated so as to form four-color separeted halftone gradation images in C (cyan), M (magenta), Y (yellow) and K (black).
- In the fields of printing and plate-making, the image scanning/reading/recording/ systems which electrically process image information of an original for forming original film plates for printing have been widely used in recent years in order to rationalize the process and improve the image quality.
- Such an image scanning/reading/recording system comprises basically an input section, a controller and an output section. More particularly, the system includes an input section where image signals are picked up by the illumination, color separation and photometric systems to convert image information photoelectrically, and the controller which processes the information in arithmetic operation such as gradation correction, color correction, contour emphasis, conversion from R (red), G (green), B (blue)→C,M,Y,K and the like depending on the plate-making conditions. The processed image information is converted into optical signals such as laser beams by the output section to be printed in the form of images on a recording medium comprising a photosensitive material. The medium is developed by a predetermined developing device and used for printing and so on as the original film plates.
- When the original to be printed is a photograph or a painting of continuous tone images, the original should be divided into dots to express the density of images. The continuous tone images are therefore transformed into halftone gradation images which are a group of dots in the sizes which are different depending on the density thereof. As a method for forming such dots, there has been proposed a method which irradiates optical signals depending on the continuous tone images on a recording medium via a contact screen placed on a film. The contact screen comprises arrays of dots with blurred circumferences. There is usually employed in the image scanning/reading/recording system a method which electrically forms a halftone screen which is equivalent to the contact screen.
- Since a good example of the prior art methods of forming a halftone screen, the method disclosed in Japanese Patent Publication (kokoku) No.49361/1977 is briefly described below.
- In FIG.1, the
reference numeral 100 denotes a basic periodic section of the halftone screen which is electrically formed. The halftone screen formed by repetition of the same pattern, and the minimum unit thereof is the basicperiodic section 100. The basicperiodic section 100 comprises eight scanning lines S₁ through S₈ arranged in parallel to each other in the direction of Y axis. The scanning lines S₁ through S₈ form respectively parts of the basic periodic 100 with unique voltage signals which change along the recording direction X. The respective voltages of the scanning lines S₁, S₂, S₄ and S₅ are set at a high level when they are passing through a point A through a point D in adot section 101 while the voltage of the scanning line S₃ is set at a low level when it passes through the point E. The voltages of the respective scanning lines S₁ through S₅ are set to gradually decrease from the points A through D toward the point E. The voltage signals at the scanning lines S₁ through S₈ may be formed into the halftone screen signals by, for example, superposing plural alternating voltage signals of a triangle shape of different periods and gradually shifting their phases for each scanning line. - When multicolor images and so on are formed into dots for reproduction, it is necessary to generate plural halftone screens and to superpose the halftone gradation images which are formed by the plural halftone screens. The halftone screens are formed respectively in the form rotated from the recording direction X at a predetermined angle ϑ in order to prevent generation of Moiré patterns at the time of superposing.
- A basic
periodic section 100 which is formed as above is generated periodically at a frequency sufficient to cover the scanning area of the original to form the halftone screen. The halftone screen signals forming such a halftone screen are superposed with the image signals which are optically read from the original at the input sectin of the image scanning/reading/recording system in order to form the halftone gradation images on the original film plate. - A halftone plate of an area modulation type for color printing is characterized by the number of screen lines (e.g. the number of lines/inch: LPI), the screen angle (m/n) and the dot patterns. The screen angle is a rational number defined by m/n of FIG.1, and is required for each of four colors of the four colors of C,M,Y and K. The screen angle and the dot patterns are formed with the method disclosed in Japanese Patent Publication No.49361/1977 mentioned above. An arbitrary number of the screen lines may easily be obtained in the drum type color scanners simply by changing the imaging magnification with optical zooming, but in the flat bed (plane) type color scanners, the optical zooming is quite difficult. In the color scanners of the flat bed type, it is necessary to optically conduct main scanning of a light spot due to its high speed processing, and it is almost impossible to further add an additinal mechanism for zooming to the system. Moreover, the control system has to be made bigger and more complex in order to change the size of the light spot or pitches.
- It is further necessary to prevent generation of a tone jump or coupling of adjacent blackened portions of the dots.
- This invention was contrived to eliminate such conventional problems and aims to provide a halftone data forming method for flat bed type color scanners which can change the number of screen lines arbitrarily without changing the scanning pitch or the size of the light spot and which can prevent generation of the tone jumps.
- According to one aspect of this invention, for achieving the objects described avobe, there is provided a method for forming halftone data which is characterized in that when an original comprising color images of a continuous tone is scanned to obtain image signals, and the image signals are superposed with halftone screen signals which are electrically generated in order to form multicolor separated halftone gradation images which are reproducible by printing, the difference in dot percentage between a first contact and a second contact in square dots is about 2% or higher.
- According to another aspect of this invention, there is privided a method for forming halftone data which is characterized in that when an original comprising color images of a continuous tone is scanned to obtain image signals, and the image signals are superposed with halftone screen signals which are electrically generated in order to form multicolor separated halftone gradation images which are reproducible by printing, the difference in dot percentage between one stage of dot where blackening starts and another stage or where whitening ends is about 2% or higher.
- The nature, principle and utility of the invention will become re apparent from the following detailed description when read in conjunction with the accompanying drawings.
- In the accompanying drawings:
- FIG.1 is a chart to show a basic periodic section of a halftone screen;
- FIGs.2 and 3 are views to show examples of patterns respectively;
- FIG.4 is a view to show an embodiment of dots with a rational tangent;
- FIG.5 is a blok diagram to show an embodiment of image recording system in structure;
- FIG.6 is a structral view to show an embodiment of a light beam scanning apparatus;
- FIG.7 is a chart to show examples of halftone data with the line number of 170 LPI;
- FIG.8 is a view to show an embodiment of a dither matrix ( dot screen signals);
- FIG.9 is a view to show a condition under which halftone data is being generated;
- FIG.10 is a chart to explain the halftone screen signals;
- FIG.11 is a block diagram to show a circuit which generates halftone gradation image signals;
- FIG.12 is a block diagram to show the detail of an address converter;
- FIG.13 is a chart to show an embodiment of the off-set table;
- FIGs.14 and 16 are charts to explain a tone jump or blackened pixels when the dither matrix shown in FIG.8 is used;
- FIGs.15,17 and 18 are views to describe operations when the dither matrix according to this invention is used;
- FIG.19 and 21 are charts to show embodiments of a dither matrix obtained according to this invention, respectively; and
- FIG.20 is a view to explain a tone jump.
- According to this invention, when separated halftone gradation images of C,M,Y and K are being formed, a light spot of predetermined size is exposed to light and scanned while the number of pitches is being restricted in order to obtain an arbitrary number of the screen lines. Since the number of the screen lines is defined with the number of pitches, this invention method may easily be applied to color scanners of the flat bed type with conventional screen angles and dot patterns.
- According to this invention, halftone screen signals or data of a dither matrix are re-arrnged in order to prevent tone jumps when the images are outputted.
- The control of the number of pitches which is a presupposition of this invention will first be explained.
- It is assumed that the angles of the four plates of C,M,Y and K are selected to be 0° , 15° , 45° and 75° , and the Y-plate is allotted at the
angle 0° , and the other three plates of C,M and K are allotted at the remaining three angles respectively. It is known emperically that the Moiré pattern by the Y-plate is not visually recognizable, and therefore the Moiré pattern caused by the remaining plates of C,M and K, should be dealt with. Theangles 15° and 75° are more precisely expressed at 18.4 ° and 71.6° when m/n =1/3 as shown in FIG.5 if the screen is formed by a rational tangent. Therefore, the twoangles 15 ° and 75° are symmetrical in respect of theangle 45° . - The primary Moiré pattern caused with the screens of 15° and 75° will now be discussed.
- It is assumed that a Moiré pattern of period K₁ is formed at an angle ϑ₁ in respect of the horizontal line in FIG.2 and the same Moiré pattern of the period K₂ is formed at an angle ϑ₂ in FIG.3 wherein the period of the first screen shown in FIG.2 is denoted as d₁ and that of the second screen in FIG.3 is denoted as d₂, and the angle formed between the two screens is denoted as α. The coordinate axes of x and y are plotted using the first screen as the reference. The points P₀ and P₁ are expressed in coordinates in FIG.2 as below.
p₁ = 2p₀ + d₁·cotα (3)
The inclination of a straight line ℓ₀ connecting the points P₀ and P₁ is expressed as below.
q₁ = d₁·tanα (8)
The inclination of the straight line ℓ₁ connecting the points Q₀ and Q₁ is expressed as below. - The relation between the primary Moiré pattern of 45° produced superposing of two plates ( 15° and 75° ) and the 45° screen or the third plate is discussed as the mechanism to produce a Moiré pattern. The parameters of rational tangent as shown in FIG.4 are given by (m,n,a), and if m < n, the angles corresponding to 15° and 75° respectively are tan⁻¹(m/n) and tan⁻¹(n/m). When p denotes a scanning pitch and a a constant of an integer, a square of p × a becomes the minimum unit of repetitive patterns. The numbers m, n and a are integers, and the numerical values are given in a manner to satisfy the following formula or the conditions to dissolve a single Moiré pattern disclosed in Japanese Patent Laid-open (kokai) No. 188564/1987 wherein γ is an integer.
- The primary Moiré pattern produced by the two plates of 45° ± (30° ± Δϑ) (provided, however, the
angles 15° and 75° are nominal angles) takes place precisely at 45° . The sufficient condition to prevent occurrence of the secondary Moiré pattern is the condition to the period of the primary Moiré pattern agree with the dot interval of the third plate of 45° . If the numerals n₀, m₀ ( < n₀ ), a₀ are integers, they becomes (m, n, a) = (m₀, n₀, a₀) at or close to 15° , (m, n, a) = (n₀, m₀, a₀) or symmetrical at or close to 75 ° , and (n, m, a) = (1, 1, a₁) at or close to 45° . The line interval d₁₅ at 15° becomes as expressed in equation (12) below.
d₄₅ = p·a₁/√2 (13)
The real angle ϑ₁₅ for 15° is expressed as below.
ϑ₁₅ = tan⁻¹(m₀/n₀) (14)
The real angle ϑ₇₅ at 75° is expressed as below.
ϑ₇₅ = tan⁻¹(n₀/m₀) (15)
If the primary Moiré pattern is formed with patterns of 15° and 75 ° in the direction of 45° , the following relations (16) and (17) are obtained.
α=ϑ₇₅-ϑ₁₅ (16)
d₁= d₂ = d₁₅ (17)
By substituting these in the above formula (5), the period K of the Moiré pattern formed will be;
K = d₄₅ (19)
As the formula (20) holds, the relation holds as expressed by a equation (21).Table 1 nominal line number 45° a , real line number 15° / 75° m/n, a, real line number real angle 65 LPI a =50, 63.9 LPI 1/3 a=100, 71.4LPI, 18.4° / 71.6° 65 LPI a =51, 62.6 LPI 1/4 a=153, 60.8LPI, 14.0° / 76 .0° 85 LPI a =40, 79.8 LPI 1/3 a=80, 89.2LPI, 18.4° / 71.6° 100 LPI a =35, 91.2 LPI 1/3 a=70, 102.0LPI, 18.4° / 71.6° 120 LPI a =27, 118.3 LPI 5/18 a=349, 120.9 LPI, 15.5° / 74.5° 120 LPI a =26, 122.8 LPI 5/17 a=314, 127.4 LPI, 16.4° / 73.6° 133 LPI a =25, 127.7 LPI 1/3 a=50, 142.8 LPI, 18.4° / 71.6° 150 LPI a =22, 145.1 LPI 4/15 a=241, 155.4 LPI, 4.9° / 75.1° 175 LPI a =20, 159.6 LPI 1/3 a=40, 178.5 LPI, 18.4° / 71.6° 175 LPI a =17, 187.8 LPI 1/4 a=51, 182.5 LPI, 14.0° / 76.0° - Based on the above formula (21), the number of screen lines usable at the scanning pitch p = 11.25 µm is calculated, and the result is shown in Table 1. Those marked with asterisk "* " represents Moiré patterns with relatively longer periods while the rest represent the conditions where no Moiré pattern is formed. Even with the conditions marked with "* " , they would not present a significant problem in practice as their period is long. Especially on the lines of 150 and 120, there is no combination which does not form Moiré pattern at the scanning pitch of 11.25 µ. The number of screen lines of 150 and 120 lines can be obtained by using 12.5 µm as the scanning pitch or 10/9 times of 11.25 µm.
Table 2 nominal line number LPI pitch p(µm) 0 ° 45 ° 15° / 75° a₀ ℓ₀ a₄₅ ℓ₄₅ m/n angle a₁₅ ℓ₁₅ 65 22.5 17 66.4 25 63.9 1/3 18.4° / 71.6° 50 71.4 85 22.5 13 86.8 20 79.8 1/3 18.4° / 71.6° 40 89.2 100 12.5 20 101.6 30 95.8 1/3 18.4° / 71.6° 60 107.1 120 12.5 17 119.5 25 114.9 1/3 18.4° / 71.6° 50 128.5 133 11.25 17 132.8 25 127.7 1/3 18.4° / 71.6° 50 142.8 150 12.5 14 145.1 20 143.7 1/3 18.4° / 71.6° 40 160.6 175 11.25 13 173.7 20 159.6 1/3 18.4° / 71.6° 40 178.5 200 12.5 10 203.2 15 191.6 1/3 18.4° / 71.6° 30 214.2 - In the second Table 2, the coarse
screen line numbers - FIG.5 shows the structure of a system which reads images with a color scanner or the like, processes them in halftone and records them in images. More particulaly, the image signals read by an
image input section 100 are digitized by an A/D converter 101, processed for gradation conversion, contour emphasis and so on by asignal processing section 102 and inputted to adot converting section 103. The image data which are converted into dots by thedot converting section 103 are inputted to a D/A converter 104, converted into analog values and recorded in images by animage output section 105. - An light beam scanning apparatus as the
image output section 105 are described below. - FIG.6 shows the light
beam scanning apparatus 30 comprises alaser diode 32 which outputs laser beam L₁ for synchronization under the control ofLD driver 31, and alaser diode 34 which outputs laser beam L₂ for recording under the control of anLD driver 33. The laser beam L₁ for synchronization which is outputted from thelaser diode 32 is directed to agalvanometer mirror 40 via acollimator 36 and amirror 38. The laser beam L₂ for recording which is outputted from thelaser diode 34 is directed to thegalvanometer mirror 40 at an angle φ in respect of the laser beam L₁ for synchronization via acollimator 42 and themirror 38. Thegalvanometer mirror 40 deflects the laser beam L₁ for synchronization and the laser beam L₂ for recording through reflection when the mirror is vibrated at a high speed. The laser beams L₁ and L₂ which have been reflected and deflected by thegalvanometer mirror 40 are directed respectively to asynchronization signal generator 50 and animage recording section 52 via ascanning lens 48 comprising an fϑ lens. The synchronizing laser beam L₁ enters thescanning lens 48 at an incident angle of φ in respect of the optical axis and directed to the synchronizingsignal generator 50 via the peripheral side of thescanning lens 48. The recording laser beam L₂ enters thescanning lens 48 within the plane including the optical axis and directed to theimage recording section 52 via the center of thescanning lens 48. - The synchronizing
signal generator 50 includesgrids 56 which are formed with a large number ofslits 54 at uniform intervals in the scanning direction of the synchronizing laser beams L₁, and the synchronizing laser beams L₁ is directed into thegrids 56 via amirror 58. Alight condensing rod 57 is provided on the back surface of thegrids 56, and the synchronizing laser beam L₁ is directed tophotodetectors 60a and 60b on both sides of the rod via thelight codensing rod 57 and converted into electric signals. The electric signals from thephotodetectors 60a and 60b are multiplied by a PLL (Phase Locked Loop)multiplier 62 and supplied to anoutput controller 64 as a synchronizing signal. Theoutput controller 64 controls theLD driver 33 based on the synchronizing signals and image signals. As shown in Table 2, once the number of screen lines and screen angles are determined, the step proceeds to designing a dot pattern. - FIG.7 shows a dot pattern ( template or dither matrix ) of 0° with 175 line. For each of the combinations of the screen line numbers and screen angle shown in Table 2, dot patterns ( template data ) as shown in FIG.7 or in practice as shown in FIG.8 are designed and used for formation of the halftone screens. The template data are usually stored either in a floppy disc or an ROM of the
image output section 105, and at the time point when the number of screen lines and the screen angle are selected, a scanning pitch p is selected from the table and a circuit is set to be controlled with the selected pitch p. The data of the template data is read out in an RAM region. As shown in FIG.9, with thetemplate data 1 and the image signals 2,halftone data 3 are formed. Referring to FIG.9, thetemplate data 1 which are read out are compared to theimage data 2, and when theimage data 2 are larger, "1 " is outputted, and when they are smaller, "0 " is outputted. In this manner, thehalftone data 3 is obtained with which light is modulated to expose a photosensitive material to obtain a halftone plate. - The method for forming a halftone screen by the
dot converting section 103 is described below. - FIG.10 shows an example of the halftone screen signals wherein a basic
periodic section 10 of a halftone screen comprising 100-halftone data is shown. In this case, the basicperiodic section 10 is defined with the points A through D to be ahalftone section 12 at a screen angle ϑ. -
- Further, the relation below is required to hold as the basic
periodic section 10 should have halftone data of the number of an integer in the directions X and Y.
n · L=α · P (23)
wherein n·L denotes a length either in the X direction or the Y direction of the basicperiodic section 10 when a side of a square forming thehalftone section 12 in either the X direction or the Y direction is denoted as L; P denotes a width of the halftone block formed with a halftone data, and α denotes the number of the halftone data forming the basicperiodic section 10 in either the X direction or the Y direction . - The data forming the
halftone section 12 with the points A through D need to be structured with the same halftone data in order to prevent occurrence of regular patterns in reproduced images. More specifically, it is necessary to provide the halftone data of an integer between the points A and B both in the directions X and Y. If the distance between the points A and B in the direction of Y is denoted as ℓ, and γ is an integer, the relation holds as below.
ℓ=γP (24)
The similar relation can also be obtained in respect of the direction X. The distance ℓ in the above case can be expressed as below.
ℓ=L· cos²ϑ · tanϑ (25)
From the formulas (23), (24) and (25), the relation below holds.periodic section 10 can be structured by repeating generating the halftone data between the points A and B in the direction Y. When the number of halftone data forming the basicperiodic section 10 is determined so as to arrange the halftone data of an integer between the points A and B, the basicperiodic section 10 can be expressed with the halftone in an amount of γ/ α determinable by the formula (5). For instance, in the case shown in FIG.10, as the relation holds as tanϑ = 1/2 ,n = 2, γ/ α becomes 1/5, and the basicperiodic section 10 can be expressed with twenty halftone data a₀ through a₁₉ of which amount is one fifth of the data. - Based on the halftone data a₀ through a₁₉ forming the basic
periodic section 10 shown in FIG.10, the method for generating the halftone gradation image signals from the continuous tone image signal will be described. - FIG.11 shows an embodiment of a circuit which is used to generate the halftone gradation image signals. The circuit comprises two
counters address converter 18, a halftone data (dither matrix data)memory 20, aline memory 22 and abinary signal generator 24. Out of preset plural halftoneoriginal data 26 selected desirable ones depending on the level of the halftone resolution and the screen angle ϑ to be transferred and stored in thehalftone data memory 20. The counter 14 counts the clock signal φx in the main scanning direction of the continuous tone images and supplies them as an address signal X to theaddress converter 18. The counter 16 counts the clock signal φY in the auxiliary scanning direction of the continuous tone images and supplies them to theaddress converter 18 as an address signal Y. Theaddress converter 18 converts the address signals (X, Y) of the halftone data at the basicperiodic section 10 into the address signals (x, y) of the halftone data a₀ through a₁₉ stored in thehalftone data memory 20 and is structured as shown in FIG.12. More specifically, theaddress converter 18 includes aremainder calculating section 183 to which the address signal X is supplied via anadder 182 and aremainder calculating section 184 to which address signal Y is supplied. Theadder 182 is supplied with an adding signal F(Y) selected by the address signal Y from an off-set table 181. The off-set table 181 comprises plural data which are determinable by combination of the screen angle ϑ and the halftone resolution level similarly in the case of the halftoneoriginal data 26. Theremainder calculating section 183 calculates the address signal x based on the equation below.
x = MOD( X + F(Y), Nx ) (27)
wherein Nx denotes the number of the halftone data in the direction X
Theremainder calculating section 184 calculates the address signal y based on the fonmula below.
y = MOD(Y,Ny) (28)
wherein Ny denotes the number of the halftone data in the direction of Y. - The
line memory 22 is supplied with the continuous tone image signals and the clock signal φx in the main scanning direction, and the outputs from theline memory 22 and thehalftone data memory 20 are supplied respectively to thebinary signal generator 24. Thebinary signal generator 24 compares the halftone data with the image signals and outputs the results of comparison as the halftone gradation image signals. - A method to convert the continuous tone image signals into the halftone gradation image signals by using the basic
periodic section 10 shown in FIG.10 is described below. - Halftone data of various levels of the screen angle ϑ and the halftone resolution are stored in the halftone
original data 26. Out of these data, the halftone data a₀ through a₁₉ are selected and loaded at the address (x, y) in thehalftone data memory 20. The address (x, y) is set within the range (0, 0) through (9, 1). Therefore, when an address signal (X, Y) from thecounters addres converter 18, theaddress converter 18 converts the address signals (X, Y) into the address signals (x, y) of the halftone data a₀ through a₁₉. In other words, in the case of (X, Y) = (0, 0), as the adding signal F(Y) is set at "0" in the off set table 181 shown in FIG.13, the output signals from theremainder calculating section address converter 18 accesses the halftone data a₀ at the address (x,y) = (0,0) from thehalftone data memory 20 and supplies the data to thebinary signal generator 24. When the address signal (X, Y) from thecounters address converter 18 selects the halftone data a₄ out of the halftone data a₀ through a₁₉ stored in thehalftone data memory 20, and supplies the data to thebinary signal generator 24. Similarly, all of the halftone data a₀ through a₁₉ which form the basicperiodic section 10 are sequentially supplied to thebinary signal generator 24. - The
binary signal generator 24 is supplied with the continuous tone image signal based on the clock signal φx from theline memory 22 in addition to the halftone data a₀ through a₁₉. Thebinary signal generator 24 compares the halftone data a₀ through a₁₉ with the continuous tone image signals, and outputs the result of the comparison in the form of the halftone gradation image signals or ON/OFF signals. The halftone signals are converted into optical signals such as laser beams and irradiated on a film by a device shown in FIG.6 to become the halftone gradation images. - In the halftone screen or the dither matrix mentioned above, a tone jump tends to occur usually at near 50% in a square dot. This is attributable to the fact that the scanning beam has an extensive width, and therefore the tone jump tends to occur at a level from which blackened portions start to be coupled between adjacent dots. In a case where a dither matrix is used as FIG.8, the ratio of the blackened portion increases in the process FIGs.14A →14B →14C or FIGs.16A →16B →16C →16D →16E. In order to prevent the tone jump from occurring in the blackening process as above, the dither matrix is formed as shown in FIG.19 with difference in dot percentage between the first and the second contacts by first connecting one contact and then the other contact as shown in FIG.15B to form a square pattern as shown in FIG.15C. The required difference in dot percentage is about 2% or more. The dot shapes at various levels of the halftone data will become as shown in FIGs.17A through 17E when the dither matrix is used.
- In dots, the tone jump tends to occur at the level where the blackening starts in the main scanning direction. This is because due to the extensive width of the scanning beam, the dot area does not increase smoothly. When the dither matrix is used as shown in FIG.8, the tone jump were likely to occur in the process of (A1) →(A2) or (A2)→(A3) in FIG.20. In the conditions shown above, as levels are set close to each other, it is more likely to produce tone jumps. P1 through P5 in FIG.20 show respectively the minimum unit of dot structure. FIG.21 shows the dither matrix wherein the difference in dot ratio between the dots where the blackening starts in the main scanning direction or between dots where the whitening ends is 2% or higher to prevent the tone jumps. When the dither matarix of FIG.21 was used, the tone jumps tended to occur at the transitions from (B1) →(B2) and (B3)→(B4) in FIG.20, but as the levels of (B1) →(B2) and (B3) →(B4) were separated to a certain extent, the number of the tone jumps was less than the dither matrix in FIG.8 as the whole halftone gradation. As to the whitened portion, the similar fact applid only if the relation of negative vs. positive is reversed. The halftone data assumes the forms as shown in FIG.18A through FIG.18E at various levels of the halftone data when the dither matrix of FIG.21 was used. In FIGs.16 through 18, the letter (A) represents the level where the blackened pixels exist in the number of 1 to 4, (B) in the number of 1 to 5, (C) in the number of 1 to 13, (D) in the number of 1 to 19, and (E) in the number of 1 to 25.
- As is described in detail in the foregoing statement, this invention method for forming halftone data can provide a desirable number of screen lines by exposing and scanning a predetermined size of a light spot at a predetermined pitch and by controlling the number of pitches when a multi-colored separated halftone gradation images are formed by combining all of C,M,Y and K or any two or more colors thereof, and prevent occurrence of the tone jumps by re-arranging the dither matrix or screen signals there of.
- It should be understood that many modifications and adaptations of the invention will become apparent to those skilled in the art and it is inteded to encompass such obvious modifications and changes in the scope of the claims appended thereto.
Claims (4)
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
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JP258229/88 | 1988-10-13 | ||
JP63258229A JP2578947B2 (en) | 1988-10-13 | 1988-10-13 | How to create dot data |
Publications (3)
Publication Number | Publication Date |
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EP0363943A2 true EP0363943A2 (en) | 1990-04-18 |
EP0363943A3 EP0363943A3 (en) | 1990-10-17 |
EP0363943B1 EP0363943B1 (en) | 1997-01-02 |
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EP89118918A Expired - Lifetime EP0363943B1 (en) | 1988-10-13 | 1989-10-11 | Method for forming halftone data |
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US (1) | US5019896A (en) |
EP (1) | EP0363943B1 (en) |
JP (1) | JP2578947B2 (en) |
DE (1) | DE68927606T2 (en) |
Cited By (3)
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US7092127B2 (en) * | 1998-11-09 | 2006-08-15 | Silverbrook Research Pty Ltd | High data rate printer architecture and method of printing using same |
US7187469B2 (en) | 1998-11-09 | 2007-03-06 | Silverbrook Research Pty Ltd | Pagewidth inkjet printer with high data rate printer architecture |
US7372598B2 (en) | 1998-11-09 | 2008-05-13 | Silverbrook Research Pty Ltd | Pagewidth inkjet printer with foldable input tray for interface protection |
Families Citing this family (11)
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US5172248A (en) * | 1988-05-18 | 1992-12-15 | Fuji Photo Film Co., Ltd. | Method for forming halftone screen and apparatus therefor |
JP2618107B2 (en) * | 1991-03-12 | 1997-06-11 | 三田工業株式会社 | Matrix address generation device and multi-value halftoning device including the address generation device |
JPH077624A (en) * | 1991-09-09 | 1995-01-10 | Mitsubishi Paper Mills Ltd | Dot picture recording method |
JP2772504B2 (en) * | 1993-03-26 | 1998-07-02 | 大日本スクリーン製造株式会社 | Mesh image generator with variable mesh angle |
JP3241157B2 (en) * | 1993-04-19 | 2001-12-25 | 富士写真フイルム株式会社 | Dot image data correction method and image processing apparatus having correction function |
JP3305495B2 (en) * | 1994-04-28 | 2002-07-22 | キヤノン株式会社 | Image processing apparatus and image processing method |
JPH09130605A (en) * | 1995-10-31 | 1997-05-16 | Fuji Photo Film Co Ltd | Method and device for outputting dot picture |
US6208430B1 (en) * | 1998-01-07 | 2001-03-27 | Xerox Corporation | Increased functionality for Holladay halftoning |
JP2001169110A (en) * | 1999-12-10 | 2001-06-22 | Fuji Photo Film Co Ltd | Image conversion device and image conversion program storage medium |
US7511857B2 (en) * | 2004-01-26 | 2009-03-31 | Hewlett-Packard Development Company, L.P. | Halftoning method and system |
JP4756597B2 (en) * | 2005-11-09 | 2011-08-24 | 株式会社リコー | Image forming apparatus and method |
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US7092127B2 (en) * | 1998-11-09 | 2006-08-15 | Silverbrook Research Pty Ltd | High data rate printer architecture and method of printing using same |
US7168777B2 (en) | 1998-11-09 | 2007-01-30 | Silverbrook Research Pty Ltd | Feedback arrangement for a printer having a microelectromechanical printhead |
US7187469B2 (en) | 1998-11-09 | 2007-03-06 | Silverbrook Research Pty Ltd | Pagewidth inkjet printer with high data rate printer architecture |
US7188921B2 (en) * | 1998-11-09 | 2007-03-13 | Silverbrook Research Pty Ltd | Generation of print data |
US7372598B2 (en) | 1998-11-09 | 2008-05-13 | Silverbrook Research Pty Ltd | Pagewidth inkjet printer with foldable input tray for interface protection |
US7413273B2 (en) | 1998-11-09 | 2008-08-19 | Silverbrook Research Pty Ltd | Generation of print data for printing |
US7744181B2 (en) | 1998-11-09 | 2010-06-29 | Silverbrook Research Pty Ltd | Printer controller for monitoring an ink drop count |
US7796300B2 (en) | 1998-11-09 | 2010-09-14 | Silverbrook Research Pty Ltd | High-speed printing method having parallel processes |
US8118380B2 (en) | 1998-11-09 | 2012-02-21 | Silverbrook Research Pty Ltd | Printer controller for monitoring an ink drop count |
Also Published As
Publication number | Publication date |
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EP0363943A3 (en) | 1990-10-17 |
JPH02105678A (en) | 1990-04-18 |
JP2578947B2 (en) | 1997-02-05 |
EP0363943B1 (en) | 1997-01-02 |
DE68927606T2 (en) | 1997-04-24 |
US5019896A (en) | 1991-05-28 |
DE68927606D1 (en) | 1997-02-13 |
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